Not sure if this is new but I noticed that now miles accumulated during Single Race in arcade mode now add to mileage on your car from GT mode too. I know it wasn't like this before because of the amount of time I spent driving many of my hundred cars that still sport 0.0 miles on them now, but I recently purchased a few new cars within the last week and they all have mileage on them and I only test drove them and raced the AI to get a feel for them. My YouTube GTR has exactly 29 miles on it, considering I drove it nearly everyday in Arcade mode since I unlocked it back in June, seems like mileage sticks now in 1.10 no matter what mode you drive your car in.
Edit... Seems like not every car will gain mileage during Time Trial. I just used my '07 M3 which I acquired in 2013 and it acquired no mileage, I just raced it in single race and it still has 0.0 miles on it. Seems it might be cars acquired before a certain update are immune to gaining mileage i Arcade mode. I'll check my other cars from 2013 to see what other cars have this weird bug.